The need to resort to an automatic transfer between a succession of work stations, has been felt for a long time now, in order to rationalize a manufacturing, assembling, mounting or treating process and to reduce, if not eliminate completely, all human intervention at each operational step in such a process.
To reach this object, the prior art has proposed a number of solutions which can be classified in two families corresponding, respectively, to transfer machines with bound carriages and to transfer machines with free carriages.
In the first family, the machines define a trackway, set up in relation with the different work stations. Such a trackway is used for moving and guiding carriages which are interconnected in the manner of an endless chain or conveyor, driven by any suitable drive member.
Such a solution may present certain advantages in special applications in which the mass of workpieces or of loads to be transferred is quite small. In other cases, such a solution present starting and stopping problems, on account of the inertia represented by all the transfer carriages organized in the form of an endless unitary chain.
One related problem resides in the difficulty of using suitable indexing means which are, nonetheless, necessary in front of each work station, so that any elementary operation relative to every stopping phase, in relation to the supported workpieces or loads, can be performed with as much accuracy as possible.
Conceivably, this requirement is not very compatible with the organization of a large number of transfer carriages into an endless chain, in which the successive interconnections of the carriages, which are necessarily articulated, introduce a tolerance factor which is difficult to control.
The transfers, known as bound transfers, are not either a particularly adaptable technical solution since there is no possibility with such a structure, of organizing by-pass tracks or stand-by sectors, and also since the moving cycle must, necessarily, conform with the longest individual operation time of all the work stations.
Such transfers therefore have a limited output and offer but little adaptability when an intervention is necessary in the course of the general process, or when it becomes imperative to use such transfers for carrying out operations for which it was not initially designed.
The transfer machines of the second family have been designed to overcome the aforesaid necessities. Such machines comprise a trackway defining guiding and supporting rails for free and independent carriages which are moved from one work station to the other, via drive members incorporated to the trackway.
It is unquestionable that such an organization has represented a positive evolution with respect to the transfer machines of the other family. Yet, a number of disadvantages are also found with these transfer machines with free carriages.
The carriages, which are placed on the trackway, are entirely controlled by that trackway and are dependent on the driving means incorporated in said trackway. This implies that the speed of such carriages is limited because there is no damping of the acceleration and deceleration phenomena which, however, should be limited in order to reduce shocks, particularly when the transfer machine has stopped, either in front of a work station, or because the carriages are in direct abutting contact.
The means of starting off the different carriages must, necessarily, be provided throughout the length of the trackway, this requiring a large number of drive members, which increases the risks of loss of synchronization, of wear, of failure, of noise, of bulkiness and of safety.
Although such machines can be produced in modular form, they nevertheless offer little adaptability, because of the necessity of always using the same constituting elements comprising a source of power, transmission members, as well as guiding members which, in general, cannot really be arranged in any other way but the linear way. The plan configurations which can be given to the trackway are, as a result, limited or else they require the use of direction changing systems which are even more complex and which, in general, have been found to be at the origin of delicate adjustments and of breakdowns, often difficult to control.
Because the elementary modules imply making use of all the constituting members, any modification will involve a lengthy stoppage time as well as delicate and costly operations, particularly when a part of the trackway between two work stations have to be modified.
And because the drive member and the transmission elements are necessarily incorporated, the overall thickness of every modular element is great and may, in some cases, raise problems of implantation for hand-operated work stations, for which a steady efficiency is always the target.
Though the existence of free and independent transfer carriages makes it possible, when defining a general work cycle, to do away with the longest operation time of one of the work stations, the maximum moving speed, between stations, is, on the contrary, limited to the capacity of the transmission members provided in the trackway. In order to counterbalance said limited speed, it is generally admitted to have a large number of free carriages travelling on the trackway. In addition to the resultingly increased overall dimensions of the trackway, the excess number of carriages unavoidably causes a localized accumulation, for example against one carriage being stopped in working service in front of a work station. In such a case, the number of carriages blocked horizontally on the trackway, increases very considerably the friction with the transmission members and strains noticeably the motor which drives them continuously. In addition, such accumulation occurs against the carriage which is stopped against the retractable stop member of a work station. Said stop member then undergoes a great thrust, resulting from an accumulation of the elementary frictions between the transmission members and the stacked carriages. Such thrust is often the cause of a deterioration of the structure or of the stopping position of the retractable stop member.
On the whole, therefore, the transfer machines equipped with free carriages, although they bring advantages over the machines of the other family, are not yet fully satisfactory, and are currently found to raise certain unsolvable problems of incorporation in fully automatic production lines using robotized work stations which, by their very structure, experience appreciably different operating times within the same production line.
In order to reduce the number of the aforementioned disadvantages, the prior art has concentrated on developing the transfer machines of the second family. One particular French patent application FR-A-2 585 008 (85-11048) can be cited to this effect. This application describes an automatic transfer machine equipped with transfer means constituted by rails and autonomous carriages or pallets, provided with their own driving means. The autonomy-inducing means consist in an electric motor which is carried by the pallet and supplied by shoes operationally coupled with collectors carried by the rails.
Such a machine is also equipped with a centralized system for controlling the progress of the pallets by using a central unit, programmed by local passage sensors, to remote-control the supply or non-supply of electric power to the collectors.
Although this particular construction is an improvement over the previous solutions, it remains nonetheless unsatisfactory for the following reasons.
The real object is to make the machine adaptable in operation, so that it will allow the performance of industrial processes liable to certain variations, the aim there being to enable the use of the transfer machine for carrying out different operations which do not all take place automatically in succession. It would then be possible with such a machine to carry out different production programmes, for the same article or for different articles, and thus to be able to meet the demand.
In order to reach this object, it is important to find a solution to two kinds of problems.
The first one is that of a reliable self-supply of driving power, and the solution proposed by Application FR-A-2 585 008 does not fulfill the object. It is indeed a known fact that when shoes are moved on collectors, especially with a low voltage current, this causes a "charring" effect which instantly changes the conditions of transmission of the electric power. As a result, no reliable-in-time displacement of a pallet equipped as aforementioned can be guaranteed.
The second problem is that of the control of the displacement of all the pallets or carriages transporting loads from one work station to another, where operations of different durations are conducted. Application FR-A-2 585 008 has not, by centralizing the control of the overall displacement, procured a smooth running, as in fact this is frozen by the program of the central unit which merely takes into account the preprogrammed passage detections to authorize the performance of an operating sequence.
There is therefore an obvious need for a transfer machine, of the type with free carriages, which will have none of the aforementioned disadvantages, but which, on the contrary, will show an ability to adapt automatically the time of transfer between two work stations, whatever the distance separating these stations and the operating time required by each one.